One reason for making recombinant DNA is to clone—that is, to produce many identical copies of—a particular gene or other DNA sequence. We have seen the term “clone” used in the context of whole cells or organisms (see Chapters 11 and 12) that are genetically identical to one another. A gene can be cloned by inserting it into a bacterial cell such as E. coli. The bacterium is allowed to reproduce and multiply into millions of identical cells, all carrying copies of the gene. Cloning might be done to get enough DNA for sequencing and subsequent analysis, to produce a protein product in quantity, or as a step toward creating an organism with a new phenotype.

Recombinant DNA is cloned by inserting it into host cells in a process known as transformation—or transfection if the host cells are derived from an animal. (See Key Concept 13.1 for another example of transformation in bacteria.) A host cell or organism that contains recombinant DNA is referred to as a transgenic cell or organism, and the non-native DNA is called a transgene. Later in this chapter we will encounter many examples of transgenic organisms, including yeast, rice plants, and even cattle.